1. Field of the Invention
The present invention relates to a roller bearing for endless linear motion comprising a long size track rail and a slide unit which is mounted on the rail to perform a relatively endless linear motion with the rollers as rolling elements, more specificially, it relates to a roller bearing for endless linear motion with the rollers being arranged in a cross-roller mode wherein a track groove thereof is improved.
2. Description of the Prior Art
FIGS. 5 and 6 show such a roller bearing 1 for endless linear motion that belongs to the technical field of the present invention. The bearing 1 is of such construction that the rollers 4 are arranged in a cross-roller mode around the skirt portion 3 of a casing 2 having a U-shaped section to allow the roller 4 to circulate endlessly. The endless circulation path of the roller bearing 1 is composed of linear track grooves 5, 5, a return path 6 formed in parallel with the track grooves 5, 5, and arc-shaped direction changing paths which are formed at the both ends of the track goove 5 and the return path 6 and connect the track groove and the return path.
The roller 4 in a load zone which is in contact with the track groove 5 formed in the track rail 7 is led, as it takes sliding motion, into the direction changing path formed within side plates 8 fixed to the both ends of the casing. The return path 6 connected to the direction changing path is formed by V-shaped grooves 10, 10 made respectively on the outer face of the casing 2 and a circulation path cover 9; the roller 4 advances, while pushed forward, from the direction changing path to the return path 6. In the drawing, 11 is a steel holding-plate of a L-shaped section, it is provided so that the roller 4 may not fall off, even when the slide unit 12 (general term for casing 2, side plate 8, cover 9, side-face seal 13, grease nipple 14, etc.) is detached from the track rail 7. Incidentally, 15 is a bottom seal. (As regards FIG. 6, assuming that the portion including the track groove 5 in the casing 2 is partitioned off from the skirt portion 3 by the alternate long and two short dash line X--X, the partitioned portion is referred to hereinafter as "a track member".)
It is known already in the above-type roller bearing for endless linear motion that the roller behavior in the boundary portion, where the roller arranged in the cross-roller mode moves from the direction changing path to the track groove in an endless circulation path, exerts a great influence on the running performance of the bearing. That is, the configuration of the boundary portion between the load zone and the no-load zone in an endless circulation path of a cross-roller mode bearing affects greatly the running performance of the bearing.
In prior arts, there is adopted a way of forming a clearance groove having a slight inclination angle in the boundary portion. FIG. 7 shows such a way, and is a longitudinal section of the track member 16, in which a V-shaped clearance grooves 17, 17 having inclination angle .theta. are formed at the both ends of the track groove 5. A boundary line 18 is formed at the boundary between the track groove 5 and the clearance groove 17. Since the rolling elements of the bearing of this type are cylindrical rollers arranged in cross-roller mode, the angle 2.beta. of the track groove is 90.degree. (FIG. 7A: a side view seen from the direction of arrow A in FIG. 7). And the angle of the clearance groove is represented as 2.alpha. as shown in FIG. 7B (view seen from the direction of arrow B in FIG. 7).
In the prior art, an angle of groove at the boundary portion between the clearance groove and the track groove is less than 2.beta., in order to make the angle 2.alpha. of clearance groove to be the same as the angle 2.beta.(=90.degree.) of track groove. Therefore, a roller coming to said boundary portion contacts with the upper part (shoulder part) of both surfaces of said track groove, and then, said roller is lifted slightly.
Such a phenomenon mentioned above affects the behavior of said roller, and thus, the stick-slip of the bearing can not be decreased, in the prior art. As regards the irregular behavior of rollers in the boundary portion, further specific description will be given in reference to FIGS. 8-12.
FIG. 8 is a top view seen fom the direction of arrow C in FIG. 7. When angle 2.alpha. of the clearance groove is formed, for example, into the same as the track groove 2.beta., the boundary line 18 takes a shape of an inverted V-shape, that is, the apex will be of a shape projecting to the end face of the track groove. Since the boundary line 18 is of an inverted V-shape, the roller 4 which has moved in from the direction changing path to the clearance groove 17 is thrusted into the track groove 5 through either point P or P (depending upon an inclinaion of rollers arranged in the cross-roller mode). The instant state of the roller 4 which has just entered the load zone from the no-load zone is shown in FIG. 9.
In FIG. 9, a load or a contact stress is approximately equally distributed (shown by hatched portion 19) in the track groove 5 on the side of track rail 7. On the other hand, as regards the track groove 5 on the side of the track member 16, since the roller 4 enters the load zone (track groove 5) from point P as stated above, the track goove 5 on the side of the track member 16 and the roller 4 make their first contact only on the upper slant face (portion shown by point P) of the track groove 5. And the contact area is so small, as compared with that in the case of the track groove 5 on the side of the track rail 7, that distribution (hatched portion 20) of the load (contact stress) is seen as an extreme end-load applied in concentration to the portion of point P.
In consideration of such load (contact stress) distribution, behavior of the roller in the portion of the boundary between the clearance groove and the track goove will be explained in reference to FIG. 10. When the view of FIG. 9 is seen from the side of the track rail 7 (from the same direction as in FIG. 8), FIG. 10 is obtained. The roller 4 I which receives a load of the distribution as shown in FIG. 9 is pushed by roller 4 II illustrated with alternate long and two short dash line to receive a driving force in the direction of arrow D. Then the roller 4 I, which has received a concentrated load in point P, will behave in a manner to turn round into the direction of arrow E with point P as its center, upon receiving a driving force in the direction of arrow D. Through such turn-round motion, roller 4 I is affected by a force acting in a manner that it may be thrown out of the circulation path, and the skew (inclination of the roller) is increased. The present inventor has found that the phenomena acts to increase the resistance of the sliding motion of the bearing, and constitutes one of the reasons for the roller to take the irregular behavior.
Next, the reason for the irregular behavior of the roller will be explained below referring to FIGS. 11 and 12. FIGS. 11 and 12 are shown for explanation given in consideration of the behavior of the roller in the direction changing path, or the behavior of the roller entering the track groove 5, and the portion around the boundary line between the clearance groove and the track groove is enlarged in FIG. 12.
As is clear from FIG. 12, since roller 4 III and roller 4 II make contact with each other at point Q lower than the center of gravity of the roller 4 II, when the roller 4 III pushes the roller 4 II into the track groove, a force is added to act in a manner to push the roller 4 II upward (the direction of arrow F). That is, rollers in the direction changing path make contact with each other in the position closer to the center of the circular arc of the direction changing path (lower portion) than to the center of the gravity of the roller, as stated above, and this fact induces the fact that the roller will advance while inclining in the direction of going out of the direction changing path, and that skew (inclination of the roller) of the roller entering the load zone will be increased.
As regards rollers 4 I and 4 II, roller 4 I receives a concentrated load in the upper portion of the track groove of the track member 16 as explained in reference to FIGS. 9 and 10, accordingly when the roller 4 I receives an arrow force D, the roller will incline (skew) in the direction of the arrow G with a center of inclination located in the region of point where the roller 4 I receives a concentrated load. When the roller skews in the direction of arrow G, it slightly inclines to the direction of advance, and acts as if it might lock in the track groove at the points S.sub.1 and T, consequently the sliding resistance is increased, and it constitutes a reason for stick slip of the roller.
The present applicant has made a number of proposals for resolving problems of the increase of sliding resistance and the irregular behavior of the roller in the region of the boundary line between the clearance groove and the track groove, and has obtained considerable results. For example, the applicant has made a proposal in Japanese Utility Model Application No. 19487-1985, disclosing an art for forming 3-step clearances in the both end portions of the track face (FIG. 13). In that art, the boundary line 18 between the clearance groove 17 and the track goove 5 is formed rectangularly with the longitudinal direction of the track groove, as shown in FIG. 14. Since the roller moves gradually from the no-load zone to the load zone, the running displacement (vertical displacement at the time when the bearing is in slide motion) due to irregular behavior of the roller is improved. The device of the Japanese Utility Model Application No. 19487-1985 has proved to be effective in itself, but it could not much restrain the motion of the roller to spring out of the endless circulation path, failing in obtaining a bearing having expected slide resistance and running displacement. Furthermore, working of the clearance groove in the both end portions of the track groove is complicated and this involves a rise of the manufacturing cost and wide scattering of accuracy, and the performance of the bearing is unstable.
On the other hand, recently the field of utilization of this kind of bearing has been further developed, and a bearing, which has higher running performances, less running displacement and small slide resistance, has been required, nevertheless it is still difficult for the conventional art to manufacture products to meet those demands.